1. Field of the Invention
The present invention relates to a fuel-vapor emission-control system for an internal combustion engine, and more specifically to a fuel-vapor emission control system for an internal combustion engine which is capable of proper adsorption of vaporized fuel within a charcoal canister and purging of vaporized fuel from the canister to the intake system of the engine, without regard to the amount of fuel vaporized from the fuel tank of the vehicle.
2. Description of the Related Art
In general, in an internal combustion engine, a fuel-vapor emission control system is provided so that fuel does not escape into the atmosphere from the fuel tank, carburetor or other places where fuel is accumulated when the engine is stopped. This fuel-vapor emission-control system causes vapor (a gas mixture of fuel vapor and air) which flows from parts in which fuel is accumulated to be adsorbed in a canister, air being released into the atmosphere and the fuel vapor which is adsorbed in the canister being purged, using the negative pressure at the intake side of the engine while running. In such a canister, to prevent the escape of fuel vapor into the atmosphere when the vehicle is stopped, due to vapor concentration dispersion caused by the temperature difference after running the vehicle, the canister is generally provided with a diaphragm on the atmospheric release port. In addition, in a split canister having a main canister and a sub-canister, there is generally a diaphragm in the path therebetween.
However, when vaporized fuel which has been adsorbed in a canister is purged to the intake manifold of the engine, there are cases in which it is possible, depending upon the operating conditions, to have a large amount of purging. Even if a large amount of purging is done, however, it is not possible to achieve a sufficient flow of air passing through the canister because of the diaphragm. A problem arises in a case such as this in that, because the amount of fuel adsorbed in the canister tends not to be reduced, the working capacity of the canister drops.